Whenever the fuel tank or the fuel line of an engine is exposed to low ambient temperatures, the fuel, or certain additives or impurities in the fuel, may gel or "wax". That is, the viscosity of the fuel, additives, or impurities increases to such a degree that the fuel no longer flows well through the fuel system. For example, the diesel fuel in the fuel tanks or fuel lines of diesel trucks may wax if the truck is driven or parked in extremely cold weather. Since waxing can interfere with the proper flow of fuel to the engine, it frequently results in poor engine performance. Severe waxing may even block the flow of fuel entirely, stalling the engine.
In addition, fuel frequently becomes contaminated with water. Sometimes water contaminants are already present in fuel when the fuel is added to the fuel tank. Improper handling or poor refining may introduce water contaminants to fuel. Moreover, water condensation inside a fuel tank sometimes takes place following outside air intrusion into the fuel tank. At low temperatures, water contamination can take the form of ice crystals suspended in the fuel. Fuel contaminated with water or ice crystals also causes poor engine performance or stalling.
Waxing may be eliminated by heating the fuel. Also, heating the fuel melts ice crystals and tends to cause water contaminants to precipitate from the fuel. Although heating the fuel may eliminate any waxing or icing, the water, in liquid form, may collect within the fuel system, for example, inside the fuel tank or inside a fuel filter disposed downstream from the fuel tank.
There have been many proposed devices for heating fuel to eliminate waxing and icing, to precipitate the water, and to purge water from the fuel system. Many such devices employ tubes which circulate a heatable fluid, for example, a fluid bearing waste heat, such as engine coolant, motor oil, or exhaust gas, through a reservoir of fuel. The fuel is then heated via a heat exchange process. However, frequently a considerable period of time is required for the exhaust gas, coolant, or oil to become hot and for the hot gas, coolant, or oil to heat the fuel sufficiently to melt ice crystals suspended in the fuel and to cause water to precipitate from the fuel. This problem is especially severe for large fuel reservoirs. Further, the fuel may not be effectively maintained in contact with the heat exchanger to ensure that the fuel is efficiently heated. Consequently, the engine may perform poorly for a long period of time following a cold start.
Another undesirable characteristic of many conventional fuel heating and water separating devices has to do with the placement of the device. First, physical contact between the water separating device and a mounting surface to which the device is mounted causes considerable conductive heat loss, disadvantageously lengthening the time required for heating the fuel sufficiently to eliminate waxing, melt any ice crystals present in the fuel, and cause water to precipitate from the fuel. Second, this type of water separating device is inflexible regarding placement and takes poor advantage of available space. Depending on the overall configuration of the vehicle in which the fuel system is installed, the water separating device might be very difficult to get at, or it might be in the way of other engine components that might require easy access for periodic maintenance.